In traditional high energy particle accelerators there has been used a distributed non-evaporable getter (NEG) system for reaching the ultimate vacuum in the system. This consists of coating a metallic non-evaporable getter on one or both surfaces of a supporting strip and then disposing the strip along substantially the whole length of the vacuum chamber constituting the beam tube of the accelerator. See U.S. Pat. No. 3,620,645 and C. Benvenuti and J-C. Decroux, Proceedings of the 7th International Vacuum Congress (Dobrozemsky, Vienna, 1977) p. 85.
Unfortunately there are disadvantages related to the use of non-evaporable getters in that they have to undergo a heating process to make them active and sorb unwanted gases, including hydrogen. Such heating can place an unacceptably high thermal load on the cryogenic cooling systems associated with the accelerator. Furthermore when the NEG is cooled down to the cryogenic temperature its sorption capacity becomes limited to the surface area only of the sorbing material with a consequent reduction of its ability to sorb hydrogen.
In addition as the accelerator reaches higher and higher circulating beam energies synchrotron radiation becomes more important as it tends to stimulate desorption of gas from the inner wall of the beam tube. In accelerators where superconducting magnets are used this gas is essentially hydrogen, with a very small amount of CO.